This invention was made during the performance of Contract No. DE-AC03-90SF18495 with the United States Department of Energy by Westinghouse Electric Corporation, Pittsburgh, Pa.
This invention relates to nuclear reactors and more particularly to a passive cooling system for the steel containment vessel used to house the nuclear reactor, the steam generators and associated equipment. In U.S. Pat. No. 5,049,353 issued Sep. 17, 1991 to Westinghouse Electric Corporation, there is described a passive containment cooling system used to cool the steel containment vessel of a nuclear power plant in the event of a failure of any of the active cooling systems used in the water cooled reactor. The cooling system described in this patent can also be used as a passive heat sink to remove all core decay heat from the steel containment vessel safely and rapidly and prevent any excessive pressure build-up within the steel containment vessel.
As further described in the above-mentioned U.S. Pat. No. 5,049,353, the passive containment cooling system uses a combination of natural air circulation between the steel containment vessel and the concrete shield building and gravity fed cooling water to provide the necessary cooling and heat sink. In the event of a failure of any of the primary active reactor cooling systems or an accident, the passive containment cooling system can be utilized to provide sufficient cooling to allow for a safe and orderly shutdown of the reactor.
The gravity fed cooling water used in this passive containment cooling system is derived from a large (400,000) gallon annular tank built into the roof of the concrete shield building that encloses the steel containment vessel and when needed, the water is allowed to flow by gravity all over the outside surfaces of the steel containment shell. Because the water is drained by the natural force of gravity, pumps and human operator actions are not necessary to provide the necessary cooling effect. The cooling water storage tank is designed to provide the cooling water requirement for several days after which additional water could be added to the tank or, as most likely, the air cooling provided would be sufficient to remove any residual heat from the steel containment vessel.
In order to provide the most efficient heat removal from the outside surface of the steel containment vessel, it is important that the passive cooling water flows over the entire surface in a thin uniform film to prevent any hot spots occurring on the surface and to maximize the cooling effect of the water. Even, uniform distribution of water over the surface of the steel containment vessel has been difficult due to the fact that the dome of the steel containment vessel is an ellipsoidal dome with a curved surface and the steel surface, while as smooth as steel fabrication practices permit, still contains a number of surface deviations and roughened areas that cause the cooling water to "channel" or otherwise not flow over the surface in a thin uniform film. These surface deviations and variations are large compared to the thickness of the desired water film. It has also been observed that if "channeling" occurs near the top of the steel containment vessel, the channel will continue on down the side of the vessel and thereby create a large area that will not be effectively cooled and lead to hot spots.
Prior attempts to provide for a uniform film of cooling water on the curved surface of the steel containment vessel have improved, but not completely solved the problem. As described in the above-mentioned U.S. Pat. No. 5,049,353, one approach to the problem was to coat the outside surface of the steel containment vessel with a special heat conductive wettable zinc-based paint. While this paint improved the flow of the cooling water as compared to a bare steel surface, channeling and hot spots still occurred.